This invention relates to liquid crystal compositions, liquid crystal display devices comprising said liquid crystal compositions and those of an OCB mode. For convenience, the liquid crystal composition of the present invention is sometimes expressed herein merely as the composition. Further, the compound represented by the formula (I) is sometimes expressed as Compound (I), and the compounds represented by other formulas are each sometimes expressed in this manner.
Practical application of a liquid crystal display device using an OCB (optically compensated bend) mode has been studied, because of its broad viewing angle and reduced response time. Recently, a liquid crystal composition having more reduced response time and being suitable for the OCB mode has been demanded so as to cope with an animation.
For conventional liquid crystal display devices of TFT (thin film transistor) drive, speeding up has been attempted using a TN (twisted nematic) mode, an IPS (in-plain switching) mode or a VA (vertical alignment) mode. However, displaying an animation as in CRT (cathode-ray tube) requires more reduced response time. Thus a liquid crystal composition and a liquid crystal display device with more reduced response time have been desired. The same applies to a liquid crystal display device of an OCB mode, for which a liquid crystal composition with a reduced response time is needed. Further, low power consumption in liquid crystal display devices has been attempted. To drive the liquid crystal display device at low voltage, a liquid crystal composition having low threshold voltage has been also desired.
The liquid crystal display devices of an OCB mode are disclosed in JP-A 7-84254 and Society for Information Display International Symposium (SID ""93), 277 (1993). The liquid crystal compositions used in the liquid crystal display device of an OCB mode are disclosed in JP-A 9-176645 and JP-A 11-302653. The liquid crystal display devices of a reflection type R-OCB mode are disclosed, for example, in Society for Information Display International Symposium (SID ""96), 618 (1996).
An object of the invention is to provide a liquid crystal composition having reduced response time and low threshold voltage and a liquid crystal display device using said composition, especially that of an OCB mode.
We have found that the liquid crystal composition of the present invention as described below has more reduced response time and lower threshold voltage as compared with conventional ones, the present composition can be suitably used in a liquid crystal display device, and that in particular a liquid crystal display device of an OCB mode. We have completed the present invention based on these findings.
The present invention resides in the following items 1 to 8.
(1) A liquid crystal composition which comprises as a first component at least one compound selected from the group of compounds represented by the formula (I) and as a second component at least one compound selected from the group of compounds represented by the formula (II), wherein the content of the first component is 30 to 95% by weight and the content of the second component is 5 to 70% by weight, based on the total weight of the composition 
wherein R1 and R3 are independently an alkyl of 1 to 10 carbon atoms and any xe2x80x94CH2xe2x80x94 in the alkyl may be replaced by xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94; R2 is an alkyl of 1 to 10 carbon atoms, xe2x80x94F or xe2x80x94Cl; X1, X2, X3, X4, X5, X6, X7, X8 and X10 are independently xe2x80x94F or xe2x80x94H; X9 is xe2x80x94F, xe2x80x94Cl, xe2x80x94CF3, xe2x80x94OCF3 or xe2x80x94OCF2H; A1 and A2 are independently 1,4-cyclohexylene, pyrimidine-2,5-diyl or 1,4-phenylene in which any xe2x80x94H may be replaced by xe2x80x94F; Z1 and Z3 are independently a single bond, xe2x80x94(CH2)2xe2x80x94 or xe2x80x94(CH2)4xe2x80x94; Z2 is a single bond or xe2x80x94CF2Oxe2x80x94; and Z4 is a single bond, xe2x80x94COOxe2x80x94 or xe2x80x94CF2Oxe2x80x94.
The meaning of the expression xe2x80x9cany xe2x80x94CH2xe2x80x94 in the alkyl may be replaced by xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94xe2x80x9d as defined herein is illustrated by the following examples, to which the present invention is not limited. For the case of the alkyl being C4H9xe2x80x94, examples of the alkyl in which xe2x80x94CH2xe2x80x94 is replaced by xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94 include C3H7Oxe2x80x94, CH3xe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94, CH3xe2x80x94Oxe2x80x94CHH2Cxe2x95x90CHxe2x80x94(CH2)3xe2x80x94, CH3xe2x80x94CHxe2x95x90CHxe2x80x94(CH2)2xe2x80x94 and H2Cxe2x95x90CHxe2x80x94(CH2)2shown in these examples, the term xe2x80x9canyxe2x80x9d denotes xe2x80x9cat least onexe2x80x9d. In view of the stability of the compound, CH3xe2x80x94Oxe2x80x94CH2xe2x80x94Oxe2x80x94 wherein oxygens are not adjacent each other is more preferable than CH3xe2x80x94Oxe2x80x94Oxe2x80x94CH2xe2x80x94 wherein oxygens are adjacent each other.
(2) The liquid crystal composition according to item 1, which further comprises as a third component at least one compound selected from the group of compounds of the formulas (III-1) and (III-2) 
wherein R4, R5 and R6 are independently an alkyl of 1 to 10 carbon atoms and any xe2x80x94CH2xe2x80x94 in the alkyl may be replaced by xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94; R7 is an alkyl of 1 to 10 carbon atoms, an alkoxy of 1 to 10 carbon atoms, xe2x80x94F, xe2x80x94Cl or xe2x80x94OCF3; A3 is 1,4-cyclohexylene or 1,4-phenylene in which any xe2x80x94H may be replaced by xe2x80x94F; Z5 is a single bond, xe2x80x94(CH2)2xe2x80x94, xe2x80x94COOxe2x80x94 or xe2x80x94CF2Oxe2x80x94, and X11 and X12 are independently xe2x80x94F or xe2x80x94H.
(3) The liquid crystal composition according to item 1 wherein the first component is at least one compound selected from the group of compounds of the formulas (I-1) and (I-2) 
wherein R1, A1, Z2, R2, X4 and X5 each have the same meanings as defined in item 1.
(4) The liquid crystal composition according to item 1, wherein the second component is at least one compound selected from the group of compounds of the formulas (II-1), (II-2) and (II-3) 
wherein R3, X6, X8, X9, X10, A2 and Z3 each have the same meanings as defined in item 1.
(5) The liquid crystal composition according to item 2, wherein the content of the third component is 2 to 30% by weight based on the total weight of the composition.
(6) An application of the liquid crystal compositions according to any one of items 1 to 5, to a liquid crystal display device of an OCB mode.
(7) A liquid crystal display device containing a liquid crystal composition according to any one of items 1 to 5.
(8) A liquid crystal display device of an OCB mode containing a liquid crystal composition according to any one of items 1 to 5.
The effects achieved by each component in the present composition are explained below.
Compound (I) has relatively large optical anisotropy and high clearing point. This compound contributes to an increase in optical anisotropy and an elevation in a phase transition temperature of a nematic-isotropic liquid (clearing point: TNI). Preferred examples of Compound (I) are shown below. 
In the above compounds, R1 has the same meaning as defined in item 1. Preferable R1 includes an alkyl of 1 to 10 carbon atoms, an alkenyl of 2 to 11 carbon atoms or an alkoxy of 1 to 9 carbon atoms. Especially preferable R1 is an alkyl of 2 to 7 carbon atoms.
Compound (II) has large dielectric anisotropy, and middle or large optical anisotropy. This compound increases the dielectric anisotropy of the composition without appreciable reduction in the optical anisotropy. Preferable examples of Compound (II) are shown below. 
In the above compounds, R3 has the same meaning as defined in item 1. Preferable R3 includes an alkyl of 1 to 10 carbon atoms, an alkenyl of 2 to 11 carbon atoms or an alkoxy of 1 to 9 carbon atoms. Especially preferable R3 is an alkyl of 2 to 7 carbon atoms.
Compound (III-1) and Compound (III-2) have low viscosity and low clearing point. These compounds are used for adjusting the clearing point of the composition, lowering the viscosity of the composition and improving the compatibility at low temperatures in the composition. Preferable examples of Compound (III-1) and Compound (III-2) are shown below. 
In the above formulas, Rxe2x80x2 is an alkyl of 1 to 10 carbon atoms or an alkoxy of 1 to 10 carbon atoms; R4, R5 and R6 have the same meanings as defined in item 2 above. Preferable R4, R5 and R6 include an alkyl of 1 to 10 carbon atoms or an alkenyl of 2 to 11 carbon atoms. Especially preferable R4, R5 and R6 are an alkyl of 2 to 7 carbon atoms or an alkenyl of 2 to 7 carbon atoms.
When the composition consists essentially of the first component and the second component, the first component is 30 to 95% by weight and the second component is 5 to 70% by weight, based on the total weight of the composition. When the composition consists essentially of the first component, the second component and the third component, it is preferable that the first component is 30 to 95% by weight, the second component is 5 to 70% by weight and the third component is 2 to 30% by weight, based on the total weight of the composition. More preferably, the first component is 40 to 85% by weight, the second component is 15 to 60% by weight and the third component is 2 to 20% by weight, based on the total weight of the composition. It is desirable that each component comprises at least two compounds.
The composition may be mixed with other liquid crystalline compounds than the first, second and third components as mentioned above, unless they adversely influence the effect of the invention. Suitable additives may be added to the composition to adjust the physical properties. For the purpose of inducing a helical structure to a liquid crystal to adjust its twist angle, a chiral dopant such as cholesteryl nonanoate (CN) or the like may be added to the composition. The composition is prepared by mixing each component and heating the mixture to dissolve each other.
The above-mentioned composition is especially suitable for the liquid crystal display device of an OCB mode. The composition can also be used in the liquid crystal display devices of TN and IPS modes. Further, the composition can be used in the liquid crystal display devices of guest host mode and polymer dispersed liquid crystal.